1. Field of the Invention
This invention relates to controlling corrosion in acidic hot hydrocarbons. More particularly, this invention relates to compositions and methods for inhibiting naphthenic acids induced corrosion of iron-containing metal alloys in hot hydrocarbons.
2. Background Art
It is widely known in the art that the processing of crude oil in its various fractions may lead to damage of iron-containing metal surfaces of the processing equipment. This corrosion is frequently associated with, in particular, the presence and activity of naphthenic acids. The corrosion occurs when the amount of naphthenic acids in the hydrocarbon reaches some critical value indicated by total acid number (“TAN”), expressed as milligrams of potassium hydroxide required to neutralize the acids in a one-gram sample. Older literature uses a rule of thumb that a TAN greater than 0.5 is required for a crude oil to cause naphthenic acid corrosion, but more recent experience indicates that the critical value can vary considerably from this value. When elevated temperatures are applied to the crude, such as the 175° C. (−347° F.) to about 400° C. (−752° F.) temperatures customarily used to refine and distill the oil, the corrosion problem is typically further exacerbated.
While various corrosion inhibitors are known in the art, the efficacy of any particular corrosion inhibitor is generally known to be dependent upon the circumstances under which it is used. As a result, a variety of corrosion inhibitors have been developed and targeted for use for treating particular crudes, for protecting particular metals, for inhibiting specific types of corrosion, and/or for use under particular conditions of temperature, environment, and the like. For example, U.S. Pat. No. 3,909,447 describes certain corrosion inhibitors as useful against corrosion in relatively low temperature oxygenated aqueous systems, such as water floods, cooling towers, drilling muds, air drilling and auto radiator systems. That patent also notes that many corrosion inhibitors capable of performing in non-aqueous systems and/or non-oxygenated systems perform poorly in aqueous and/or oxygenated systems. The reverse is true as well. The fact that an inhibitor that has shown efficacy in oxygenated aqueous systems does not suggest that it would show efficacy in a hydrocarbon. Moreover, the fact that an inhibitor has been effective at relatively low temperatures does not indicate that it would also be effective at elevated temperatures. In fact, it is common for inhibitors that are very effective at relatively low temperatures to become ineffective at temperatures such as the 175° C. (−347° F.) to 400° C. (−752° F.) temperatures encountered in oil refining. At such temperatures, corrosion is notoriously troublesome and difficult to alleviate. Thus, U.S. Pat. No. 3,909,447 contains no teaching or suggestion that it would be effective in non-aqueous systems such as hydrocarbon fluids, especially hot hydrocarbon fluids, nor is there any indication in that patent that the compounds disclosed therein would be effective against naphthenic acid induced corrosion at elevated temperatures.
As commonly used, naphthenic acid is a collective term for certain organic acids present in various crude oils. Although minor amounts of other organic acids may also be present, it is understood that the majority of the acids in a naphthenic acid based crude are naphthenic in character, i.e., with a saturated ring structure that conforms to a formula such as one of the following:
In the above formulas, m is typically 1-2, and n varies. It is basically any carboxylic acid group with at least one saturated 5 or 6 membered ring attached. One simple example is cyclopentanoic acid.
The molecular weight of naphthenic acid can extend over a large range. However, the majority of the naphthenic acid in crude oils is found, after distilling, in the lighter fractions, including, for example, gas oil. When hydrocarbons containing such naphthenic acid contact iron-containing metals, especially at elevated temperatures, severe corrosion problems arise.
Various approaches to controlling naphthenic acid induced corrosion have included neutralizing and/or removing the naphthenic acids from the crude being processed; blending low acid number oils with more corrosive high acid number oils to reduce the overall neutralization number; and using relatively expensive corrosion-resistant alloys in the construction of the crude's processing apparatus. These attempts are generally disadvantageous in that they require additional processing and/or add substantial cost to treatment of the crude oil. Alternatively, U.S. Pat. No. 4,443,609 discloses certain tetrahydrothiazole phosphonic acids and esters as being useful additives for inhibiting acid corrosion. Such inhibitors can be prepared by reacting certain 2,5-dihydrothiazoles with a dialkyl phosphite. While these tetrahydrothiazoles phosphonic acids or esters offer good corrosion inhibition, they tend to break down under high temperature conditions.
Another disadvantage to using phosphorus-based compounds as corrosion inhibitors is that the phosphorus has been alleged to impair the function of various catalysts used to treat crude oil, such as in fixed-bed hydrotreaters and hydrocracking units. Thus, crude oil processors are often faced with a dilemma, since corrosion itself, if not inhibited, may result in accumulation in the hydrocarbon fluid of a catalyst-impairing amount of iron, as high as 10 to 20 ppm in some cases. Unfortunately, while there are a number of commercially available non-phosphorus-based inhibitors, they are known to be generally somewhat less effective than the phosphorus-based compounds.
A significant advance in phosphorus-based naphthenic acid induced corrosion inhibitors is reported in U.S. Pat. No. 4,941,994. Therein it is disclosed that metal corrosion in hot acidic liquid hydrocarbons in inhibited by the presence of a corrosion inhibiting amount of a dialkyl and/or trialkyl phosphite with an optional thiazoline. Another patent, U.S. Pat. No. 5,863,415, discloses that thiophosphorus compounds of a specific formula are particularly useful for corrosion inhibition in hot liquid hydrocarbons and may be used at concentrations that add to the fluid less of the catalyst-impairing phosphorus than some of the previous phosphorus-based corrosion inhibitors. These thiophosphorus compounds also offer the advantage of being able to be prepared from relatively low cost starting materials.
In view of the above, it would be desirable in the art to find additional methods and compositions for inhibiting or controlling naphthenic acid induced corrosion in crude oils, particularly at elevated temperatures, that do not suffer from the drawbacks of the prior art.